This invention relates to a method of manufacturing a curved discharge tube, and more particularly a curved discharge tube, for example a ultra high pressure mercury lamp utilized to form a pattern with photolithography technique by irradiating a photosensitive substance with ultraviolet rays.
When forming such a fine pattern as an integrated circuit pattern on the surface of a semiconductor wafer by photolithography, a curved discharge tube, for example a curved ulta high pressure mercury lamp is used for the purpose of minimizing the effect caused by the interference or diffraction of light.
As shown in FIG. 1A such curved discharge tube comprises a curved transparent tube 1 made of glass or quartz, for example, inert gas 2 such as mercury and argon and tungsten electrodes 3 sealed in the glass tube at the opposite ends. When the electrodes 3 are connected across a source of a predetermined voltage the central portion of the tube luminesces. Such a curved discharge tube is utilized as a light source having little diffraction and interference of light and can form an image of small aberration. The light emitted by the tube is projected upon a semiconductor wafer through an optical system including concave and convex lenses, a reflecting mirror etc. and then projected upon a photoconductor wafer through another optical system. Where the light emitted by the discharge tube is focused and then passed through a fine slit made of a vapor deposited chromium film to form a beam having small scattering, only a portion of the light emitted by the tube, that is the light between dot and dash lines shown in FIG. 1 passes through the slit so that the dimension of that portion of the tube should be accurate. The birds eye view of the brightness distribution shown in FIG. 1 is obtained by observing the light emitted by a portion as of the curved discharge tube shown in FIG. 1 through a pin hole. The crest of the brightness lies at the central portion of the discharge arc. Especially in a double tube type ultra high pressure mercury arc lamp the vest lies at substantially the center of the inner tube. Accordingly, in order to improve the quality of the light source, the diameter D of the outer tube D, the diameter d of the inner tube and the radius of curvature of the axis of the tube should be accurate.
Usually the curved mercury lamp is prepared in the following manner. At first a straight tube 6 having a uniform wall thickness as shown in FIG. 2a is heated and then pressed against a curved jig to form a curved tube as shown in FIG. 2b. At this time, the outer side of the tube is elongated, whereas the inner side is compressed with the result that the wall thickness on the outer side is decreased whereas that on the inner side is increased. For this reason the inner opening of the tube becomes eccentric. Since the glass tube is heated to 1600.degree.-200.degree. C. there is a large temperature difference between the tube and the jig, whereby a large strain remains in the curved tube. Furthermore, due to the pressure applied by the jig. the inner opening of the tube would be deformed. Thus, in order to make uniform the distribution of the light emitted by the discharge arc it is necessary to make uniform the inner and outer diameters of the curved tube.